Conventionally, increasing melt tensile strength of a resin has been thought to be effective for improving foam-moldability. Therefore, polypropylene resins have been designed so as to increase strain hardening capability by providing a branched molecular structure to cause the molecules to be entangled and by causing high molecular weight components to be included in the resins.
Although providing strain hardening capability is effective for improving foam-moldability, conventional methods of providing strain hardening capability have following problems. First, as the method for producing a propylene-based resin with a branched structure, a method of copolymerizing a non-conjugated diene and propylene (Patent Document 1) and a method of irradiating the propylene-based polymer with electron beams (Patent Documents 2 to 6) are commonly known.
However, it is difficult to control the branched structure in the former method, resulting in a problem of a poor outward surface appearance of a foam-molded product due to gel production. The latter method requires installation of special equipment in the propylene polymer manufacturing plant, which results in an increase in the production cost. In addition, product properties may unduly change due to changes in the polymer structure during recycling in these methods.
Next, as the method for providing high molecular weight components, a method of blending components having greatly differing molecular weights (Patent Document 7) and a multistage polymerization method using a continuous polymerization unit can be given. The former method requires secondary processing to ensure sufficient dispersion of high molecular weight components and to maintain an excellent outward surface appearance of foam-molded products. The latter method includes a method of providing the high molecular weight components in a preliminary polymerization stage (Patent Documents 8 to 11) and a method of providing the high molecular weight components in the polymerization stage. In the method of providing the high molecular weight components in a preliminary polymerization stage, not only the amount of high molecular weight components that can be added is limited, but the increase in melt tension is also limited.
On the other hand, the method for providing a high molecular weight propylene-based polymer by multistage polymerization has been used as a common method, because the method does not require special manufacturing equipment and can be operated with easy quality control. A method for expanding the molecular weight distribution by an organosilicon compound using a catalyst carrying MgCl2 in the multistage polymerization has been proposed (Patent Documents 12 to 14). However, the effect of improving melt tension was insufficient.
A method for continuously providing a multistage propylene-based polymer in two or more polymerization vessels using a catalyst carrying MgCl2, while controlling the hydrogen concentration in the system has been proposed (Patent Documents 15 to 19). However, the method is impractical, because the polymerization temperature must be extremely low in order to provide an ultrahigh molecular weight propylene-based polymer having an intrinsic viscosity [η] of 10 dL/g or more in tetralin at 135° C. In the case of providing a polymer having an intrinsic viscosity [η] of less than 10 dL/g, the effect of improving melt tension was insufficient.
In addition, when a foamed sheet is produced from a propylene-based polymer or a composition of the propylene-based polymer, the product must have an excellent outward surface appearance, particularly controlled corrugated marking properties, which requires controlled viscoelastic characteristics of the propylene-based polymer composition.
[Patent document 1] Japanese Patent Application Laid-open No. 06-080729
[Patent Document 2] Japanese translation of PCT application No. 2002-542360
[Patent Document 3] Japanese Patent Application Laid-open No. 2000-309670
[Patent Document 4] Japanese Patent Application Laid-open No. 2000-336198
[Patent Document 5] Japanese Patent Application Laid-open No. 2002-012717
[Patent Document 6] Japanese Patent Application Laid-open No. 2002-363355
[Patent Document 7] Japanese Patent Application Laid-open No. 2002-309049
[Patent Document 8] Japanese translation of PCT application No. 2002-509575
[Patent document 9] Japanese Patent Application Laid-open No. 10-279632
[Patent Document 10] Japanese Patent Application Laid-open No. 11-315178
[Patent Document 11] Japanese Patent Application Laid-open No. 2000-143866
[Patent Document 12] Japanese Patent Application Laid-open No. 2001-247616
[Patent Document 13] Japanese Patent Application Laid-open No. 2001-048916
[Patent Document 14] Japanese Patent Application Laid-open No. 2001-055413
[Patent Document 15] Japanese Patent Application Laid-open No. 59-172507
[Patent Document 16] Japanese Patent Application Laid-open No. 05-239149
[Patent Document 17] Japanese Patent Application Laid-open No. 07-138323
[Patent Document 18] Japanese Patent Application Laid-open No. 11-228629
[Patent Document 19] Japanese Patent Application Laid-open No. 2000-226478
The invention has been achieved in view of the above-described problems. An object of the invention is to provide a multistage propylene-based polymer possessing high melt tension and outstanding viscoelastic characteristics, a method for producing the same, and a propylene-based resin composition.